HR: 15:45h
AN: NS14A-02    [Abstracts]
TI: Near-Surface Geophysical Investigations of the Green Valley and Hayward Faults
AU: * Kimball, M A
EM: mindy.kimball@us.army.mil
AF: California State University East Bay Department of Geological Sciences, 25800 Carlos Bee Boulevard, Hayward, CA 94542
AU: Craig, M S
EM: craig@csuhayward.edu
AF: California State University East Bay Department of Geological Sciences, 25800 Carlos Bee Boulevard, Hayward, CA 94542
AU: Heller, S J
EM: heller4@llnl.gov
AF: California State University East Bay Department of Geological Sciences, 25800 Carlos Bee Boulevard, Hayward, CA 94542
AU: Lienkaemper, J J
EM: jlienk@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025
AB: We conducted seismic refraction and ground-penetrating radar (GPR) surveys at two active right-lateral strike-slip fault zones in the San Francisco Bay Area in order to gain a better understanding of near-surface geology and to provide data needed to constrain fault slip rates. Seismic refraction data were recorded using a 24-channel seismograph with a sledgehammer source. GPR data were recorded using 50 MHz antennae and 0.5 m trace spacing. The site on the Green Valley fault was selected due to its geomorphic expression of classic linear valleys and offset stream channels. The primary geophysical targets are buried paleochannels that can serve as piercing points for the determination of fault offset. Seismic refraction data indicate three layers in the near-surface zone: an uppermost layer ~1 m thick, with seismic P-wave velocity (V_P) ~100 m/s; a second layer 5-7 m thick, with V_P ~600 m/s; and the top of the third layer at 8-10 m depth, with V_P ~1700 m/s. We also recorded a grid of several GPR lines at this site. Some of the GPR lines appear to show paleochannel features at ~2-6 m depths. We will attempt to sample these features with an auger. Cone Penetrometer Test (CPT) holes were logged at several locations along two of the lines where seismic and GPR data were recorded. This site will be trenched within the next year to obtain paleoseismic data that can be used to better constrain the slip rate of the fault. The second site is Tyson's Lagoon, a sag pond between parallel strands of the southern Hayward fault. This site has been extensively trenched with the goal of determining the slip history of the fault. Detailed trench logs, borehole logs and CPT logs are available. Preliminary refraction data indicate a top layer typically 6-10 m thick with V_P ~300 m/s, underlain by material with V_P ~1900 m/s. The top layer reaches its maximum thickness at ~10 m depth in the central portion of Tyson's Lagoon, between the fault traces, and shallows to ~1 m on the west side of the western fault trace. Two GPR lines that we recorded at this site appear to image the same surface detected by seismic refraction. The 1968 borehole and 2002 CPT logs clearly indicate the presence of a surface of significant material contrast at similar depths, and this surface has been inferred to be the contact between Pleistocene gravels and overlying Holocene deposits. We interpret the surface detected at similar depths through seismic and GPR to be the same contact. None of the trenches in Tyson's Lagoon were deep enough to detect this surface in the central portion of the pond. In this study, we use GPR and seismic refraction as complementary techniques in support of paleoseismologic investigations: GPR often provides a more detailed subsurface image, while seismic refraction is less susceptible to cultural disturbances and provides better depth control. We continue to refine our processes for employing these two geophysical methods together in an attempt to 1) determine optimal locations for proposed trenches, 2) correlate geophysical data with ground-truth data (trench, CPT, borehole), and ultimately 3) develop "virtual trenching" methods that use geophysical techniques to extend real trenches beyond their actual length and depth dimensions.
DE: 0935 Seismic methods (3025)
DE: 1744 Tectonophysics
DE: 7221 Paleoseismology
DE: 7223 Seismic hazard assessment and prediction
SC: Near-Surface Geophysics [NS]
MN: 2005 Joint Assembly